The present invention relates to a magnetic random access memory and an initializing method thereof, and more particularly to a domain wall motion-type magnetic random access memory and an initializing method thereof.
A magnetic random access memory (MRAM) of a domain wall motion-type memory cells is known as a type of MRAM. For example, such an MRAM of the domain wall motion-type memory cells is disclosed in Japanese Patent Publication JP 2009-99625A. FIG. 1A is a diagram schematically showing a structure of the domain wall motion type memory cell in JP 2009-99625A. The memory cell 101 of the domain wall motion type is provided with a magnetic record layer (to be also referred to as a free layer) 110, a tunnel barrier layer 120, a pin layer 130, wirings 201, 202, and 203. The magnetic record layer 110 is a ferromagnetic layer which has a perpendicular magnetic anisotropy. The pin layer 130 is a ferromagnetic layer whose magnetization is fixed. The tunnel barrier layer 120 is a non-magnetic layer provided between the magnetic record layer 110 and the pin layer 130. The magnetic record layer 110 is composed of a magnetization invertible region 113 which has an invertible magnetization and which is connected to the pin layer 130 through the tunnel barrier layer 120, a the first magnetization fixed region 111a which is connected with the magnetization invertible region 113 in a first boundary and whose magnetization has a fixed direction, and a the second magnetization fixed region 111b which is connected with the magnetization invertible region 113 in a second boundary and whose magnetization has a fixed direction. The magnetization invertible region 113, the tunnel barrier layer 120 and the pin layer 130 configure an MTJ (magnetic tunneling Junction). In the memory cell 101 of this domain wall motion type, the initialization is required in the drive. The initialization of the memory cell 101 is performed by introducing only one domain wall into the magnetic record layer 110.
Generally, there are two modes of inversion nucleation and domain wall motion in which the magnetization of the magnetic substance is inverted. In the mode of inversion nucleation, a nucleus having a magnetization in a second direction (second magnetic substance) is generated in a first magnetic substance having the magnetization in a first direction by applying a magnetic field in the second direction opposite to the first direction, and the magnetization of the first magnetic substance is inverted to the second direction through the growth of the nucleus. In the mode of domain wall motion, a domain wall which exists in a boundary between the first magnetic substance having the magnetization in the first direction and the second magnetic substance having the magnetization in the second direction moves in the first magnetic substance so that the magnetization of the first magnetic substance is inversed to the second direction, by applying a magnetic field is applied to the second direction. Here, the followings are defined as H is the applied magnetic field, Hn is a magnetic field when an inversion nucleus is generated (hereinafter, to be referred to as an inversion nucleation magnetic field), and Hd is a magnetic field when the domain wall motion occurs (hereinafter, to be referred to as a domain wall motion magnetic field). At that time, when the magnetization of the magnetic substance is inversed, the following conditions (1) or (2) must be satisfied:                condition (1): in a region, H>Hn is satisfied (magnetization inversion through inversion nucleation), and        condition (2): in the region, H>Hd is satisfied and the region is contact with the domain wall (the magnetization inversion through the domain wall motion).        
AS mentioning above, in order to initialize the domain wall motion type memory cell 101, only one the domain wall must be introduced into the magnetic record layer 110. For this purpose, the regions of the magnetic record layer 110 are sequentially subjected to magnetization inversion. At that time, the regions need to be separated into a region in which a magnetization inversion magnetic field is small so that the magnetization inversion is performed (the above (1) or (2) is satisfied) and a region in which the magnetization inversion magnetic field is large so that the magnetization inversion is not performed.
FIG. 1B shows sectional views of a magnetic record layer for a method of initializing an MRAM described in JP 2009-99625A.
First, a magnetic field (Hz1) is applied in a +Z direction so that the magnetization directions in regions of the magnetic record layer 110 are all directed to a same direction (Step 1). Next, a magnetic field Hz2 (<Hz1) in the −Z direction is applied to the magnetization invertible region 113 to satisfy Hz2>Hn (Step 2). At this time, because the magnetization invertible region 113 satisfies the condition (1), the inversion nucleus is formed in the magnetization invertible region 113. The magnetization of the magnetization invertible region 113 is inverted when the magnetic field in the −Z direction is increased (Hz3). Domain walls 112c and 112d are introduced between a first magnetization fixed region 111a and the magnetization invertible region 113 and between the magnetization invertible region 113 and a second magnetization fixed region 111b, respectively (Step 3). Since the first magnetization fixed region 111a satisfies the condition (2) when the magnetic field in the −Z direction is further increased (Hz4) so that Hz4>Hd is satisfied in the magnetization fixed region 111a, the magnetization of the first magnetization fixed region 111a is inverted (Step 4). If the applied magnetic field is returned to zero in this state, the anti-parallel initial state is realized in which only one the domain wall 112d is injected in the DW layer.